It is widely known to use a film of plasticized polyvinyl butyral obtained by adding a plasticizer to polyvinyl butyral as an interlayer of a laminated glass. The plasticized polyvinyl butyral film has excellent adhesion, light stability, transparency and low-temperature flexibility, and finds extensive use as an interlayer film for laminated glasses used in automobiles, aircraft and buildings. In many cases, after extrusion in sheet form, it is wound up on a paper tube, etc. and stored or transported as such.
Conventional interlayer films for laminated glasses have strong adhesion at their surfaces. When the film as formed is wound up or when a wound structure of the film obtained by winding is stored or transported at ambient temperature, the film adheres to itself (this property is referred to in this application as the self-adhering property), and the film undergoes deformation during unwinding and becomes useless and in an extreme case, is broken. Attempts have been made to prevent such adhesion by spraying a powdery adhesion-preventing agent such as sodium bicarbonate on to the surface of the film, or cooling the interlayer film to a temperature of 10.degree. C. or less, or interposing a non-tacky film such as a polyethylene film between layers of the film. The method involving the spraying of the adhesion-preventing agent has the disadvantage that in bonding the polyvinyl butyral film to sheets of glass, the film should be subjected in advance to a washing step and a drying step to remove the adhesion-preventing agent. The cooling method requires a cooling device from immediately after the production of the film to the time of use and during packaging, transporting and storing. Furthermore, because water droplets form on the film surface when the temperature of the cooled film is returned to ordinary temperature, a step of re-adjusting the moisture content of the film is required immediately before use. All this will add to the cost of the film. According to the method comprising interposing the non-tacky film, dust in the air tends to adhere to the interlayer film owing to static charges generated by the use of the non-tacky film.